1. Field of the Invention
This invention relates to a charge separation type heterojunction structure used in e.g., a solar cell or a light emitting diode and which has fullerene as a portion of a constituting material thereof. This invention also relates to a manufacturing method for the charge separation type heterojunction structure.
2. Description of Prior Art
Up to now, a silicon pn junction semiconductor etc has been extensively used in e.g., a solar cell or a light emitting diode. Of late, the energy conversion efficiency of the silicon pn junction semiconductor has been improved appreciably in comparison with that when the silicon pn junction semiconductor was initially devised.
Among the materials for the solar cell, there is e.g., titania in addition to silicon. Recently, however, fullerene, as a carbon compound, has attracted attention. The features of fullerene is hereinafter explained in connection with the discovery and the history of development thereof.
Fullerene is a series of carbon compounds composed only of carbon atoms, as is diamond or graphite. The existence of fullerene was confirmed in eighties. That is, it was found in 1985 in a mass analysis spectrum of a cluster beam by laser ablation of carbon. It was, however, five years later that the manufacturing method in reality was established. Specifically, a manufacturing method for fullerene (C60) by arc discharge of a carbon electrode was first found in 1990. Since then, fullerene is attracting notice as a carbonaceous semiconductor material (see Kratschmer, W., Fostiropoulos, K, Huffman D. R. Chem. Phys. Lett. 1990, 170, 167. Kratschmer, W. Lamb L. D., Fostiropoulod. K, Huffman, D. R. Nature 1990, 347,354).
Fullerene is a spherical carbon Cn (n=60, 70, 76, 78, 80, 82, 84, . . . ) which is a molecular aggregate resulting from spherical aggregation of an even number not less than 60 of carbon atoms. Representatives of the fullerenes are C60 with 60 carbon atoms and C70 with 70 carbon atoms. Of these, the C60 fullerene is of a polyhedral structure termed truncated-icosahedron obtained from an icosahedron by truncating each of the twelve vertices. Hence, each vertex is replaced by a pentagon. Thus, the C60 fullerene has a molecular structure of what may be termed a soccer ball type in which its 60 apices are all occupied by carbon atoms. On the other hand, C70 has what may be termed a rugby ball type molecular structure.
In a C60 crystal, C60 molecules are arranged in a face-centered cubic structure. It has a band gap of approximately 1.6 eV and may be deemed as a semiconductor. In an intrinsic state, it has an electrical resistivity of approximately 107 Ωcm. It has a vapor pressure of approximately 1 m Torr at 500° C. and, on sublimation, is capable of vapor depositing a thin film. Not only C60 but other forms of the fullerene are readily vaporized in vacuum or under reduced pressure and hence are able to yield an evaporated film easily.
However, the molecules of fullerene forms, such as C60 or C70, the most mass-producible, are of zero dipole moment, such that evaporated films produced therefrom are fragile in strength, because only the van der Waal's force acts between its molecules. Thus, if the evaporated film is exposed to air, molecules of oxygen or water tend to be diffused and intruded into the gap between the fullerene molecules (FIG. 2), as a result of which the evaporated film is not only deteriorated in structure but adverse effects may be occasionally produced in its electronic properties. This fragility of the fullerene poses a problem in reference to device stability when applying the fullerene to fabrication of a thin-film electronic device.
For overcoming the weak points the fullerene polymer film, described above, the method of producing a so-called fullerene polymer consisting in polymerizing fullerene molecules has been proposed. Typical of these methods is a method of forming a fullerene polymer film by light excitation [see (a) Rao, A. M., Zhou, P, Wang., K. A, Hager., G. T., Holden, J. M., Wang, Y., Lee, W. T., Bi, X, X., Eklund, P. C., Cornet, D. S., Duncan, M. A., Amster, J. J. Science 1993, 256995, (b) Cornet, D. C., Amster I. J.,Duncan, M. A., Rao A. M., Eklund P. C., J. Phys. Chem. 1993, 97,5036, (c) Li. J., Ozawa, M., Kino, N, Yoshizawa, T., Mitsuki, T., Horiuchi, H., Tachikawa, O; Kishio, K., Kitazawa, K., Chem. Phys. Lett. 1994, 227, 572].
In these methods, in which light is illuminated on a previously formed evaporated fullerene film, numerous cracks tend to be formed in the film surface due to volumetric contraction produced on polymerization, so that produced films are problematic in strength. Moreover, it is extremely difficult to form a uniform thin film of a large surface area.
It has also been known to apply pressure or heat to fullerene molecules or to cause collision of fullerene molecules against one another. It is however difficult to produce a thin film, even though it is possible to form a film (see, for a molecule collision method, (a) Yeretzian, C., Hansen, K., Diedrich, F., Whetten, R. L., Nature 1992, 359,44, (b) Wheten, R. L., Yeretzian, C., Int. J. Multi-layered optical disc. Phys. 1992, B6,3801, (c) Hansen, K., Yeretzian, C., Whetten, R. L., Chem. Phys. Lett. 1994, 218,462, and (d) Seifert, G., Schmidt, R., Int. J. Multi-layered optical disc. Phys. 1992, B6,3845; for an ion beam method, (a) Seraphin, S., Zhou, D., Jiao, J. J. Master. Res. 1993, 8,1995, (b) Gaber, H., Busmann, H. G., Hiss, R., Hertel, I. V., Romberg, H., Fink, J., Bruder, F., Brenn, R. J. Phys. Chem., 1993, 97,8244; for a pressure method, (a) Duclos, S. J., Brister, K., Haddon, R. C., Kortan, A. R., Thiel, F. A. Nature 1991,351,380, (b) Snoke, D. W., Raptis, Y. S., Syassen, K. 1 Phys. Rev. 1992, B45, 14419, (c) Yamazaki, H., Yoshida, M., Kakudate, Y., Usuda, S., Yokoi, H., Fujiwara, S., Aoki, K., Ruoff, R., Malhotra, R., Lorents, D. J., Phys. Chem. 1993, 97,11161, and (d) Rao, C. N. R., Govindaraj, A., Aiyer, H. N., Seshadri, R. J. Phys. Chem. 1995, 99,16814).
Noteworthy as a fullerene polymerization method or film-forming method, which should take the place of the above-enumerated fullerene polymerization methods, is the plasma polymerization method or the micro-wave (plasma) polymerization method, previously proposed by the present inventors in e.g., Takahashi, N., Dock, H. or in Matsuzawa, N., Ata M. J., Appl. Phys. 1993, 74,5790. The fullerene polymer film, obtained by these methods (see FIGS. 3 and 4), are thin films produced by polymerization of the fullerene molecules through an electronic excited state. It is appreciably increased in strength in comparison with the evaporated thin fullerene film, dense and high in pliability. Since the fullerene polymer film is scarcely changed in its electronic properties in vacuum or in air, it may be premeditated that its dense thin-film properties effectively suppress diffusion or intrusion of oxygen molecules into the inside of the film. In reality, generation of fullerene polymer consisting the thin film by these methods may be demonstrated by the time-of-flight mass spectrometry.
Irrespective of the type of the plasma method, electron properties of a fullerene polymer film possibly depend appreciably on its polymerization configuration. In reality, the results of mass spectrometry of the C60 polymer film, obtained by the micro-wave plasma method, bear strong resemblance to those of the C60 argon plasma polymer thin film, previously reported by the present inventors [see Ata, M., Takahashi, N., Nojima, K., J. Phys. Chem. 1994, 98, 9960, Ata, M., Kurihara, K., Takahashi, J. Phys., Chem., B., 1996, 101,5].
The structure of the fullerene polymer may be estimated by the pulse laser excited time-of-flight mass spectrometry (TOF-MS). In general, there is known a matrix assist method as a method for non-destructive measurement the high molecular polymer. However, lacking the solvent capable of dissolving the fullerene polymer, it is difficult to directly evaluate the actual molecular weight distribution of the polymer. Even with the mass evaluation by Laser Desorption Ionization Time-of-Fight Mass Spectroscopy (LDITOF-MS), it is difficult to make correct evaluation of the mass distribution of an actual fullerene polymer due to the absence of suitable solvents or to the reaction taking place between C60 and the matrix molecule.
The structure of the C60 polymer can be inferred from the profile of a dimer or the peak of the polymer of LDITOF-MS, as observed in the ablation of such a laser power as not to cause polymerization of C60. For example, LDITOF-MS of a C60 polymer film, obtained with a plasma power of e.g., 50 W, indicates that the polymerization of C60 molecules is most likely to take place through a process accompanied by loss of four carbon atoms. That is, in the mass range of a dimer, C120 is a minor product, whilst C116 is produced with the highest probability.
According to semi-empiric C60 dimer calculations, this C116 may be presumed to be D2h symmetrical C116 shown in FIG. 10. This may be obtained by C58 recombination. It is reported that this C58 is yielded on desorption of C2 from the high electronic excited state including the ionized state of C60 [(a) Fieber-Erdmann, M., et al., Phys. D. 1993, 26,308 (b) Petrie, S. et al., Nature 1993,356,426 and (c) Eckhoff, W. C., Scuseria, G. E., Chem. Phys. Lett. 1993, 216,399].
If, before transition to a structure comprised of two neighboring five-membered rings, this open-shell C58 molecules are combined with two molecules, C116 shown in FIG. 10 is produced. However, according to the notion of the present inventors, it is after all the [2+2] cycloaddition reaction by the excitation triplex mechanism in the initial process of the C60 plasma polymerization. The reaction product is shown in FIG. 9. On the other hand, the yielding of C116 with the highest probability as mentioned above is possibly ascribable to desorption of four sp3 carbons constituting a cyclobutane of (C60)2 yielded by the [2+2] cycloaddition from the excited triplet electronic state of C60 and to recombination of two C58 open-shell molecules, as shown in FIG. 6.
If a powerful pulsed laser light beam is illuminated on a C60 fine crystal on an ionization target of TOF-Ms, as an example, polymerization of fullerene molecules occurs through the excited electronic state, as in the case of the micro-wave plasma polymerization method. At this time, ions of C58, C56 etc are also observed along with peaks of the C60 photopolymer.
However, since no fragment ions, such as C582+ or C2+ are observed, direct fragmentation from C603+ to C582+ and to C2+, such as is discussed in the literature of Fíeber-Erdmann, cannot be thought to occur in this case. Also, if C60 is vaporized in a C2F4 gas plasma to form a film, only addition products of fragment ions of F or C2F4 of C60 are observed in the LDITOF-MS, while no C60 polymer is observed. Thus, the LDITOF-MS, for which no C60 polymer is observed, has a feature that no C58 nor C56 ions are observed. These results of observation support the fact that C2 loss occurs through a C60 polymer.
The next problem posed is whether or not the C2 loss is directly caused from 1, 2-(C60)2 produced by the [2+2] cycloaddition reaction shown in FIG. 6. Murry and Osawa et al proposed and explained the process of structure relaxation of 1, 2-(C60) 2 as follows [(a) Murry, R. L. et al, Nature 1993, 366,665, (b) Strout, D. L. et al., Chem. Phys. Lett. 1993, 214,576, Osawa, E, private letter].
Both Murry and Osawa state that, in the initial process of structure relaxation of 1, 2-(C60)2, C120(d) of FIG. 13 is produced through C120(b) of FIG. 11, resulting from cleavage of the 1, 2-C bond, having the maximum pinch of the cross-linked site, from C120(c) of FIG. 12 having the ladder-like cross-linking by Stone-Wales transition (Stone, A. J., Wales, D. J., Chem. Phys. Lett. 1986, 128, 501, (b) Saito, R. Chem. Phys. Lett. 1992, 195,537). On transition from C120(c) of FIG. 12 to C120(b) of FIG. 11, energy instability occurs. However, on further transition from C120(c) of FIG. 12 to C120(d) of FIG. 13, the stabilized state is restored.
Although it is not clear whether the nC2 loss observed in the polymerization of C60 by plasma excitation directly occurs from 1, 2-(C60) thought to be its initial process or after certain structure relaxation thereof, it may be premeditated that the observed C118 assumes the structure shown in FIG. 14 by desorption of C2 from C120(d) of FIG. 13 and recombination of dangling bonds. Also, C116 shown in FIG. 15 is obtained by desorption of two carbon atoms of the ladder-like cross-linking of C118 of FIG. 14 and recombination of bonds. Judging from the fact that there are scarcely observed odd-numbered clusters in the dimeric TOF-MS, and from the structural stability, it may be presumed that the loss in C2 is not produced directly from 1, 2-(C60) 2, but rather that it is produced through C120(d) of FIG. 13.
Also, Osawa et al states in the above-mentioned literature that D5d symmetrical C120 structure is obtained from C120(a) through structure relaxation by multi-stage Stone-Wales transition. However, insofar as the TOF-MS of the C60 polymer is concerned, it is not the structure relaxation by the multi-stage transition reaction but rather the process of structure relaxation accompanied by C2 loss that governs the formation of the polymer by plasma irradiation.
In a planar covalent compound in general, in which a π-orbital crosses the σ-orbital, spin transition between 1(π−π*)-3(π−π*) is a taboo, while it is allowed if, by vibration-electric interaction, there is mixed the a-orbital. In the case of C60, since the π-orbital is mixed with the σ-orbital due to non-planarity of the π covalent system, inter-state crossing by spin-orbital interaction between 1(π−π*) and 3(π−π*) becomes possible, thus producing the high photochemical reactivity of C60.
The plasma polymerization method is applicable to polymerization of C70 molecules. However, the polymerization between C70 molecules is more difficult to understand than in that between C60. Thus, the polymerization is hereinafter explained in as plain terms as possible with the aid of numbering of carbon atoms making up C70.
The 105 C—C bonds of C70 are classified into eight sorts of bonds represented by C(1)-C(2), C(2)-C(4), C(4)-C(5), C(5)-C(6), C(5)-C(10), C(9-C(10), C(10)-C(11) and C(11)-C(12). Of these, C(2)-C(4) and C(5)-C(6) are of the same order of double bond performance as the C═C in C60. The π-electrons of the six members of this molecule including C(9), C(10), C(11), C(14) and C(15) are non-localized such that the C(9)-C(10) of the five-membered ring exhibit the performance of the double bond, while the C(11)-C(12) bond exhibits single bond performance. The polymerization of C70 is scrutinized as to C(2)-C(4), C(5)-C(6), C(9)-C(10) and C(10)-C(11) exhibiting the double-bond performance. Meanwhile, although the C(11)-C(12) is substantially a single bond, it is a bond across two six-membered rings (6, 6-ring fusion). Therefore, the addition reaction performance of this bond is also scrutinized.
First, the [2+2] cycloaddition reaction of C70 is scrutinized. From the [2+2] cycloaddition reaction of these five sorts of the C—C bonds, 25 sorts of dimers of C70 are produced. For convenience of calculations, only nine sorts of the addition reactions between the same C—C bonds are scrutinized. Table 1 shows heat of the reaction (ΔHf0(r)) in the course of the process of yielding C140 from C70 of two molecules of the MNDO/AN-1 and PM-3 levels.
In the table, ΔHf0(r)AM-1 and ΔHf° (r)PM-3 means calculated values of the beat of reaction in case of using parameterization of the MNDO method which is a semi-empirical molecular starting method by J. J. Stewart.
TABLE 1ΔHf0(r)luster(kcal/ΔHf0(r)bond(referencemol)(kcal/mol)lengthdrawing)AM-1PM-3cross-linking(Å)C140(a)−34.63−38.01C(2)-C(2′), C(4)-C(4′)1544(FIG. 15)C(2)-C(4), C(2)-C(4′)1607C140(b)−34.33−38.00C(2)-C(4′), C(4)-C(2′)1544(FIG. 16)C(2)-C(4), C(2′)-C(4′)1607C140(c)−33.94−38.12C(5)-C(5′), C(6)-C(6′)1550(FIG. 17)C(5)-C(6), C(5′)-C(6′)1613C140(d)−33.92−38.08C(5)-C(6′), C(6)-C(5′)1551(FIG. 18)C(5)-C(6), C(5′)-C(6′)1624C140(e)−19.05−20.28C(9)-C(9′), C(10)-C(10′)1553(FIG. 19)C(9)-C(10), C(9′)-C(10′)1655C140(f)−18.54−19.72C(9)-C(10′), C(10)-C(9′)1555(FIG. 20)C(9)-C(10), C(9′)- C(10′)1655C140(g)+3.19−3.72C(10)-C(10′), C(11)-C(11′)1559(FIG. 21)C(10)-C(11), C(10′)-C(11′)1613C140(h)+3.27−3.23C(10)-C(11′), C(11)-C(10′)1560(FIG. 22)C(10)-C(11), C(10′)-C(11′)1613C140(i)+64.30+56.38C(11)-C(11′), C(12)-C(12′)1560(FIG. 23)C(11)-C(12), C(11)-C(12′)1683
In the above Table, C140(a) and (b), C140(c) and (d), C140(e) and (f) and C140(g) and (h) are anti-syn isomer pairs of the C(2)-C(4), C(5)-C(6), C(9)-C(10) and C(10)-C(11) bonds, respectively. In the addition reaction between C(11) and C(12), only D2h symmetrical C140(i) is obtained. These structures are shown in FIGS. 15 to 23. Meanwhile, an initial structure of a C70 polymer by the most stable [2+2] cycloaddition is shown in FIG. 14.
From this Table 1, no energy difference is seen to exist between the anti-syn isomers. The addition reaction between the C(2)-C(4) and C(5)-C(6) bonds is as exothermic as the addition reaction of C60, whereas that between the C(11)-C(12) is appreciably endothermic. Meanwhile, the C(1)-C(2) bond is evidently a single bond. The heat of reaction of the cycloaddition reaction in this bond is +0.19 and −1.88 kcal/mol at the AM-1 and PM-3 level, respectively, which are approximately equal to the heat of reaction in C140(g) and (h). This suggests that the cycloaddition reaction across the C(10) and C(11) cannot occur thermodynamically. Therefore, the addition polymerization reaction across the C70 molecules occurs predominantly across the C(2)-C(4) and C(5)-C(6), whereas the polymerization across the C(9)-C(10) bonds is only of low probability, if such polymerization takes place. It may be premeditated that the heat of reaction across the C(11)-C(12), exhibiting single-bond performance, becomes larger than that across the bond C(1)-C(2) due to the appreciably large pinch of the cyclobutane structure of C140(i), in particular the C(11)-C(12) bond. For evaluating the effect of superposition of the 2p2 lobe of sp2 carbon neighboring to the cross-1 inking bondage at the time of [2+2] cycloaddition, the values of heat generated in the C70 dimer, C70—C60 polymer and C70H2 were compared. Although detailed numerical data are not shown, it may be premeditated that the effect of superposition can be safely disregarded across C140(a) to (h), insofar as calculations of the MNDO approximate level are concerned.
The mass distribution in the vicinity of the dimer by the LDITOF-MS of the C70 polymer film indicates that dimers of C116, C118 etc are main products. Then, scrutiny is made into the structure of C136 produced on desorbing four carbon atoms making up cyclobutane of a dimer (C70) 2, as in the process of obtaining D2h-symmetrical C116 from C60 and recombining remaining C68. These structures are shown in FIGS. 28 to 36. Table 2 shows comparative values of the generated heat (ΔHf0) of C136.
In Table 2, ΔHf0 AM-1, ΔHf0 PM-3, cross-linking and the binding length are the same as those of Table 1.
TABLE 2ΔHf0(r)luster(kcal/ΔHf0(r)bond(referencemol)(kcal/mol)lengthdrawing)AN-1PM-3cross-linking(Å)C136(a)−65.50−61.60C(1)-C(8′), C(3)-C(5′)1.351(FIG. 24)C(5)-C(3′), C(8)-C(1′)1.351C136(b)−64.44−61.54C(1)-C(3′), C(3)-C(1′)1.351(FIG. 25)C(5)-C(8′), C(8)-C(5′)1.351C136(c)00C(4)-C(13′), C(7)-C(10′)1.352(FIG. 26)C(10)-C(7′), C(13)-C(4′)1.352C136(d)+0.09+0.11C(4)-C(7′), C(7)-C(4′)1.351(FIG. 27)C(10)-C(13′), C(13)-C(10′)1.354C136(e)+112.98+102.89C(5)-C(8′), C(8)-C(5′)1.353(FIG. 28)C(11)-C(14′), C(14)-C(11′)1.372C136(f)+69.47+59.44C(5)-C(14′), C(14)-C(5′)1.358(FIG. 29)C(11)-C(8′), C(8)-C(11′)1.352C136(g)−3.74−9.20C(5)-C(15′), C(15)-C(5′)1.344(FIG. 30)C(12)-C(9′), C(9)-C(12′)1.352C136(h)+2.82−5.30C(5)-C(9′), C(9)-C(5′)1.372(FIG. 31)C(12)-C(15′), C(15)-C(12′)1.334C136(i)+98.50+84.36C(13)-C(10′), C(15)-C(16′)1.376(FIG. 32)C(10)-C(13′), C(16)-C(15′)1.376
It is noted that C136(a) to (i) are associated with C140(a) to (i), such that C(2) and C(4), which formed a cross-link at C140(a), have been desorbed at C136(a). It is noted that carbon atoms taking part in the four cross-links of C136(a) are C(1), C(3), C(5) and C(8), these being SP2 carbon atoms. Among the dimers shown in Table 1, that estimated to be of the most stable structure at the PM-3 level is C140(c). Therefore, in Table 2, ΔHf0 of C136(c), obtained from C140(c), is set as the reference for comparison. It may be seen from Table 2 that the structures of C136(a) and C136(b) are appreciably stabilized and that C136(e), C136(f) and C136(i) are unstable. If the calculated values of ΔHf0 of per a unit carbon atom of the totality of C140 and C136 structures are evaluated, structure relaxation in the process from C140 to C136 only take place in the process from C140(a) and (b) to C136(a) and (b). Thus, the calculations of the MNDO approximation level suggest that, in the C70 cross-link, not only are the sites of the [2+2] cycloaddition of the initial process limited to the vicinity of both end five-membered rings traversed by the main molecular axis, but also is the cross-link structure of the π-covalent system, such as C136, limited to C136 obtained from the dimer of C70 by the cycloaddition reaction across C(2)-C(4) bond. The molecular structure of more stable C136, yielded in the process of relaxation of the structure shown in FIG. 13, is shown in FIG. 14.
The polymer film of C60 shows semiconducting properties with band gap evaluated from temperature dependency of the dark current being of the order of 1.5 to 2 eV. The dark conductivity of the C60 polymer film obtained with the micro-wave power of 200 W is on the order of 10−7 to 10−8 S/cm, whereas that of the C70 polymer film obtained for the same micro-wave power is not higher than 10−13 S/cm, which is approximate to a value of an insulator. This difference in the electrical conductivity of the polymer films is possibly attributable to the structures of the polymer films. Similarly to the sole cross-link bond in which two-molecular C60 is in the state of open-shell radical state, the cross-link of a dimer of 1, 2-C(60) due to [2+2] cycloaddition reaction of FIG. 1 is thought not to contribute to improved electricaly conductivity. Conversely, the inter-molecular cross-link, such as C116, forms the π-covalent system, and hence is felt to contribute to improved electrically conductivity. The cross-link structures of C118, C114 and C112, now under investigations, are thought to be a π-covalent cross-link contributing to electrically conductivity.
It may be contemplated that the electrical conductivity usually is not increased linearly relative to the number of electrically conductive cross-links between fullerene molecules, but is changed significantly beyond the permeation limit at a certain fixed number. In the case of C70, the probability of the [2+2] cycloaddition reaction is presumably lower than that in the case of C60, while the structure relaxation to the electrically conductive cross-linked structure such as that from C140 to C136 can occur only on specified sites. In light of the above, the significant difference in electrically conductivity between the two may possibly be attributable to the fact that, in the C60 polymer film, the number of cross-links contributing to electrically conductivity is large and exceeds the permeation limit, whereas, in the case of C70, the permeation limit is not exceeded because of the low probability of polymerization and limitation of formation of electrically conductive cross-links.
Taking into account the discovery of the fullerene molecule, its evaporated film and fullerene polymer film and the mechanism of polymerization thereof, discussed in the foregoing, we return to the discussion of the solar cell referred to in the beginning part of the present specification.
The material fullerene has latent possibility of yielding a solar cell improved both economically and as to physical characteristics. As a matter of fact, several solar cells having fullerene as its constituent material have so far been proposed (see JP Patents Nos.9656473, 95230248 and 99325116, U.S. Pat. No. 5,171,373 and WO 9405045).
However, the solar cells, hitherto proposed, are common in exploiting the fullerene evaporated film, so that the above-mentioned problem attributable to the fragility of the evaporated film, in particular the durability or physical properties of electrons, as yet remains unsolved.
Meanwhile, the fullerene polymer film, belonging to the fullerene system as does the above-mentioned evaporated film, exhibits sufficient durability due to its superior physical properties such as freeness from oxygen diffusion into the polymer bulk material. However, it has scarcely been attempted up to now to use the material as a constituent material for fabrication of the solar cells.
This may possibly be attributable to the circumstances that the industrial fullerene polymerization technique has been developed only recently. In addition, the fact that the method of definitely identifying the fullerene polymer film by a non-destructive technique has not been established possibly needs to be taken into consideration.
As means for clarifying the interconnection of carbon skeletons in a carbonaceous compound, there is also known a method such as a nuclear magnetic resonance method. However, insofar as carbonaceous thin film, such as a fullerene polymer film, is concerned, difficulties are encountered in measurement due to failure in definite observation of the pattern of free induction attenuation depending on electrical conductivity and to transverse relaxation to nuclear spin by dangling unpaired spin.
Moreover, the nuclear magnetic resonance method is not suited as means for monitoring structural changes in the carbonaceous thin film material due to difficulties encountered in magic angle spin of an individual sample.